In the neuroimaging literature, there is a large gap in knowledge regarding the relationship between blood oxygen level dependent signals (BOLD) that are measured and neuronal inhibition. The circuits in the basal ganglia that control the suppression and release of saccadic eye movements provide a unique opportunity to fill this gap in knowledge. We propose to study BOLD signals in response to activation of an inhibitory circuit involved in eye movement control. We will insert genes encoding the light-activated ion channel, ChR2 into an output nucleus of the basal ganglia causing them to express ChR2. A light source provided to the channel- expressing neurons will open ion channels and cause neuronal depolarization of inhibitory neurons. BOLD signals will be measured. We have one Specific Aim: to determine BOLD signals in response to activation of an inhibitory neural circuit involved in eye movement control. We will perform three experiments to achieve this aim. 1) Using the neuron-specific promoter CAG, we will express ChR2 in neurons of the substantia nigra pars reticulata (nigra) and measure BOLD responses to light activation using fMRI; 2) using inhibitory neuron-specific promoters (PV and GAD67) we will express ChR2 in inhibitory nigral neurons and measure BOLD signals in response to light activation using fMRI; 3) we will record the electrical activity (LFPs) in the super colliculus and the thalamus (targets of nigral inhibition) in response to light activation to compare BOLD responses to electrical responses. The results of these experiments will uncover critical mechanisms of neural processing of inhibition and how this fundamental neuronal activity appears in BOLD signals.